1. Field of the Invention
The present invention relates to improved methods of delivering radioisotopes to tumor cells for effecting targeted radioimmunotherapy (RAIT). More specifically, the present invention relates to a radioimmunoconjugate wherein an alpha- or beta-emitting radioisotope is complexed to a binding agent attached to a fragment of an immunoglobulin such as Fab′. A clearing agent such as D-lysine may be administered to the patient along with the radioimmunoconjugate so that targeted tumor or cancer cells are destroyed, but damage to organs and tissues is minimized. After administering a radioimmunoconjugate to a patient, bone marrow or autologous stem-cells may be grafted to the patient.
2. Related Art
One therapeutic method used in cancer treatment involves directing antibodies carrying a therapeutic agent or cytotoxic compound to the diseased tissues. When localized at the disease site, the antibody delivers the therapeutic agent or cytotoxic compound to the cancerous cells. One approach to this methodology involves delivering radioisotopes to the diseased cells. This approach has proven useful in diagnosis where a radioisotope with particular imaging properties is delivered to the targeted diseased tissue.
Several methods have been used in radioimmunotherapy (RAIT). In one method, a radioisotope with desirable properties is carried by an antibody to a diseased tissue with a corresponding antigen. Various immunoglobulins such as IgG and IgM have been used to carry radioisotopes to an antigen located on a targeted disease tissue.
Various radioisotopes have been used in RAIT. 212Bi and its parent 212Pb have been successfully chelated to antibodies and other proteins via DTPA (diethylenetriamine pentaacetic acid) and DOTA (tetraazacyclododecane-N,N′,N″, N′″-tetraacetic acid) derivatives and used as alpha emitting radioisotopes in RAIT. For example, Macklis et al. disclose a radioimmunoconjugate directed against a murine antigen present on the surface of malignant T-cells. Science, 240, 1024, 1988. The radioimmunoconjugate disclosed by Macklis et al. includes a 212Bi complexed to a cyclic anhydride of DTPA attached to a monoclonal antibody. Science, 240, 1024, 1988. One drawback to the use of 212Bi and its parent 212Pb in RAIT is that the decay product of 212Bi is 209Tl which is a high-energy β and γ-emitter. This characteristic raises radiation protection problems.
Another alpha-emitting radioisotope that has been investigated in immunotherapy is 211At. Bloomer et al. Science, 212, 340, 1988. The half-life of 211At of 7.2 hours is significantly longer than that of 212Bi which has a half-life of only 60.55 minutes. It is known that 211At undergoes 42% alpha decay and 58% electron capture. The alpha particle emitted upon decay has an energy of 5.94 MeV whereas electron capture produces 80 keV of γ radiation. The 211Po daughter arising from electron capture has a half-life of 0.5 seconds and decays by emitting an alpha particle with an energy of 7.43 MeV. Thus, by either rouite one obtains one high energy alpha particle per 211At decay. Isotopically pure 211At is accessible by electron capture of 211Rn which can be isolated in radiochemically pure form from irradiated thorium by gas chromatography. Chalkin et al. Chemiker-Ztg., 101, 470, 1977.
Astatine is a halogen atom and thus behaves similarly to iodine. It has long been known that blood clearance of 211At is very rapid because the astatine is quickly accumulated in the thyroid gland. Hamilton et al. Proc. Nat'l. Acad. Sci., 26, 483, 1940. In fact, studies performed on rats and monkeys in the 1950's confirmed that the concentration of astatine in the thyroid is at least twvo orders of magnitude higher than in other organs, except for the stomach. Hamilton et al. Univ. Calif. Pubi. Pharmacol., 2, 283, 1954.
Another radioisotope that has been investigated in conjunction with RAIT is 213Bi. This radioisotope is known to decay mainly (98%) by β and 440 keV γ emission with a half-life of 45.6 minutes to the ultra-short lived high-energy (8.375 MeV) alpha-emitter 213Po (t1/2 of 4 μs), whereas a direct alpha-decay pathway to 209Tl plays only a negligible role (2% of all 213Bi decays).
U.S. Pat. No. 5,641,471 issued to Geerlings discloses a method for preparing 213Bi for therapeutic use. In the disclosed method a monoclonal antibody is used as a targeting moiety. A chelator such as CHX-DTPA (cyclohexyldiethylenetriamine pentaacetic acid) is attached to the antibody and functions to chelate the radioisotope. In this manner, the radioisotope is delivered to the target cell where it can function in a therapeutic manner to destroy the diseased tissue.
U.S. Pat. No. 5,246,691 issued to Geerlings et al. discloses radioimmunotherapy using alpha particle emissions. Specifically disclosed, is the use of 225Ac and its daughters as part of an immunoconjugate also comprising an antibody such as human monoclonal antibody and humanized antibodies. The cited patent discloses that chelating agents are used to bind actinium and bismuth radioisotopes in the radioimmunoconjugate comprising a radionuclide that emits alpha particles, a chelating agent, and a slowly localizing antibody such as a human IgM antibody. Also disclosed is the use of scavenging agents such as DTPA, EDTA, PLED, and crown ethers for use in binding wandering isotopes and thus prohibiting the isotopes from invading non-targeted organs and tissues.
U.S. Pat. No. 5,428,154 issued to Gansow et al. discloses a chelate comprising a DOTA derivative and a metal including Pb, Bi, Y, and the lanthanides. Also disclosed is the linking of the metal chelate to a biomolecule to form a delivery system for the chelated metal. Gansow et al. discloses that both alpha and beta-emitters (212Pb, 212Bi, and 90Y) can be chelated to a DOTA derivative attached to a monoclonal antibody directed against an epitope on tumor cells.
A number of references have described radioimmunoconjugates comprising a radioisotope, and a chelator attached to a monoclonal antibody. Hovever, a need remains for a radioimmunoconjugate that utilizes a Fab′ fragment of an immunoglobulin to target diseased tissue or cancer cells. A need also remains for a method of treating a patient that minimizes the amount of damage done by unbound radioisotopes.